1. Field of the Invention
The present invention relates to a wheelchair and more particularly, to a wheel set structure of an electric wheelchair.
2. Description of the Related Art
A conventional electric wheelchair 1a, as shown in FIG. 1, comprises wheelchair frame 10a, a pair of front wheel assemblies 11a, a pair of middle wheel assemblies 12a, and a pair of rear wheel assemblies 13a. The wheelchair frame 10a has two pivot holders 102a bilaterally disposed at the front side, and an upright set stem 104a supporting a chair 14a. The front wheel assemblies 11a each have a front wheel holder 112a respectively pivotally connected to the pivot holders 102a and pivotally holding a respective front wheel 114a. The middle wheel assemblies 12a each have a middle wheel holder 122a respectively pivotally connected to the pivot holders 102a and pivotally holding a respective middle wheel 124a. The rear wheel assemblies 13a each have a rear wheel holder 132a respectively fastened to the rear side of the wheelchair frame 10a and pivotally holding a respective rear wheel 134a. 
By means of pivotally connecting the front wheel holders 112a and the middle wheel holders 122a to the pivot holders 102a, the front wheel holders 112a and the middle wheel holders 122a can be biased relative to the wheelchair frame 10a when the electric wheelchair 1a is moving over an uneven road surface.
When a user is going to sit on or to leave from the chair 14a of the electric wheelchair 1a, the user will step on a footplate 15a that is fixedly fastened to the front side of the wheelchair frame 10a. When the user steps on the footplate 15a, the user's whole body weight is allocated on the footplate 15a. If the footplate 15a receives an excessively high pressure, the wheelchair frame 10a will be forced to turn about the axis extending through the pivot holders 102a, lifting the rear side of the wheelchair frame 10a from the ground. At this time, the chair 14a may collapse, causing the user to fall to the ground or to be hurt by the chair 14a. Therefore, this prior art electric wheelchair 1a is still not safe in use, tending to cause accidental injury.